The present invention generally relates to the field of hydraulic fracturing, and more particularly, to propping compositions for use with fracturing fluids, and their use to enhance fracture conductivity.
Hydraulic fracturing is a stimulation process that can increase the rate at which oil or gas can be produced. One of the first steps in hydraulic fracturing is to introduce large amounts of fracturing fluids into a formation in order to create and/or extend fractures. These fractures allow oil or gas to travel more easily from the rock pores, where the oil or gas is trapped, to the production well. A hydraulic fracture is formed by pumping a fracturing fluid into a wellbore at a rate sufficient to increase the pressure downhole to a value in excess of the fracture gradient of the formation rock. This pressure can create a crack in the formation and allow the fracturing fluid to enter and extend the crack farther into the formation. This cracking can result in a fracture network. For example, a two winged fracture can be formed in a single vertical plane that is perpendicular to a plane along the least principle stress.
A porous material (e.g., formation rock containing hydrocarbons) may be characterized by several key parameters that affect the extraction rate and ultimate recovery of hydrocarbons. For example, each rock layer can have a certain porosity, which is the percentage of pore volume or void space within a rock that can contain hydrocarbons. This void volume is typically filled with a mixture of oil, gas, and brine water. In other words, the fractions of these three components can add up to the total saturation, which corresponds to about 100% of the void space. For example, if porosity is 25% of the total rock volume, then a 50% oil saturation would mean that 12.5% of the total rock volume is filled with oil.
Certain formations have lower effective porosity because of blockages such as gases trapped in the formation matrix, the various layers of rock, or in the bedding planes. As used herein, “effective porosity” refers to the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability. Total porosity is considered to be the total void space regardless of whether it affects fluid flow.
On one hand, the total porosity of a formation sets the theoretical upper limit for how much hydrocarbon can be recovered. In practice, it is difficult to obtain or come close to obtaining the theoretical limit because rock pores usually contain other fluids besides hydrocarbons and the actual recovery of hydrocarbons is limited by the interconnectivity of a porous material between its pores. This interconnectivity can impact the permeability, which is a measure of a porous material's ability to transmit fluid. In order to optimize the recovery of hydrocarbons, the ideal situation is to have a high porosity material whose pore spaces or void volumes are well interconnected.
Fracture acidizing is one known method of enhancing permeability. This method involves pumping acid into a formation at high enough pressures to cause the formation to crack. The acid etches certain rock faces to increase the permeability of the formation. Another method of enhancing permeability is matrix acidizing in which acids are introduced into a formation at pressures below the fracture pressure of the formation where the acid reacts with soluble substances in the formation matrix.
A potential problem in these fracturing/stimulation treatments is the closure and healing of fractures. To keep fractures open after the injection of fracturing fluid has stopped, solid proppant particulates are usually introduced. Commonly used proppant particulates include, but are not limited to, sand, resin-coated sand, ceramics, and the like. In order for proppant particulates to be effective, their load bearing strength must exceed the closure pressure of the fracture. As used herein, “closure pressure” indicates the pressure at which a fracture effectively closes. In practice, the closure pressure is not a constant value and typically increases during a fracturing operation. For example, after fracturing has taken place and production of the well has begun, pore pressures typically decrease while the stress on the proppant can increase. It is important that the proppant is able to withstand the closure pressure throughout production so that the proppant pack is a permeable conduit through which the formation fluids can flow.
With stimulation techniques being used at greater depths and higher closure pressures, there are several challenges facing current proppant technology. Under these conditions, it is far more likely that the load on each proppant grain exceeds the grain's ability to support a load, thus leading to proppant breaking. It is believed, for example, that spherical proppants face high loads due to the localization of stress at each point where the proppants contact other proppant grains and/or the formation fracture face.
Proppant debris can also lead to the obstruction of pathway. For example, when proppant particulates shatter, the pieces fill the pore spaces of the proppant bed, often resulting in huge loss of flow capacity for hydrocarbons. While certain proppant particulates such as bauxite have high load bearing strength capabilities, typically less proppant is used and much longer fractures are needed.
Traditional proppant particulates also face theoretical limitations that limit the flow of hydrocarbons through the proppant pack. A proppant pack is essentially a porous medium whose porosity is determined by the void space between the packed proppant particulates. Because of the geometrical constraints of packing, it is believed that a traditional proppant pack (e.g., spherical proppant particulates) typically cannot have a porosity greater than about 35%. This low theoretical porosity can limit the permeability and fracture conductivity of the proppant packs, which in turn, can limit the recovery of hydrocarbons.